Cell Structure & Components

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Introduction to CellsCells – the smallest units capable of carrying out life functions using. . .

Organelles – “little organs” – carry on essential functions of cellsEnzymes – direct chemical reactions in cells which mediate metabolic events within a cell.

Metabolism – the sum of all chemical reactions in the cell, may be anabolic or catabolic

The organelles, proteins and enzymes present (due to gene expression) determine a cells structure and functionAbout 210 distinct cell types in the human body

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Introduction to Cells

Cells have three main components1. Plasma membrane 2. Cytoplasm

A. OrganellesB. CytosolC. Inclusions

3. Nucleus

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Structure of a Generalized Cell

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Structure of a Generalized Cell

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The Plasma MembraneStructure of membrane

Fluid mosaic model contains:Phospholipids – contains both polar & non-polar regions

may have glucose chain attached to “head” = glycolipidGlycolipids and glycoproteins form the glycocalyx of a cell.The phospholipids will arrange in a tail-to-tail configuration due to the amphipathic nature of the molecule.

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Plasma Membrane cont…Cholesterol – provides two important functions due to its structural interactions with the tails of the phospholipids

The polar region of the cholesterol is attracted to the polar heads of the phospholipids, making the membrane resilient and less permeablewhile the tail of the cholesterol remains in the tails of the phospholipids and prevent the crystallization of the long hydrocarbon chains

Membrane proteinsIntegral proteins – firmly imbedded in, or attached to phospholipid bilayer Peripheral proteins – attach to membrane surface

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Plasma Membrane cont…

The Membrane proteins may have sugars attached to them creating

GlycoproteinsReceptors, adhesion, hemostasis

The Lipids may also have sugars attached to them creating

GlyoclipidsEnergy and cell markers (ABO)

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Plasma Membrane Surface

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The Cytoplasm

Cytoplasm –lies internal to plasma membraneConsists of cytosol, organelles, and inclusions

Cytosol (cytoplasmic matrix)Jelly-like fluid in which other cellular elements are suspendedConsists of water, ions, and enzymes

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Cytoplasmic Organelles

Generally classified as either membrane bound or non-membrane bound

Membrane Bound OrganellesEndoplasmic Reticulum, Golgi Apparatus, Mitochondria, Peroxisomes, Lysosomes, Vesicles

Non-Membrane Bound OrganellesRibosomes, cytoskeletal components

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Cytoplasmic Organelles

Endoplasmic reticulum – “network within the cytoplasm”

Rough ER – closely associated with the nuclear envelope

ribosomes stud the external surfacesSite of protein synthesis

Smooth ER – consists of tubules in a branching network

No ribosomes are attached; therefore no protein synthesisSite of lipid synthesis

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The Endoplasmic Reticulum and Ribosomes

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Cytoplasmic Organelles

Golgi apparatus – a stack of three to ten disk-shaped envelopes

Sorts products of rough ER and sends them to proper destination

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Mitochondria

Mitochondria – generate most of the cell’s energy; most complex organelle

Consists of an outer membrane and a highly convoluted inner membraneAlso contains mitochondrial DNAEnzymes present for production of energy (ATP) Figure 2.9

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Cytoplasmic Organelles

Lysosomes – membrane-walled sacs containing digestive enzymes

Digest unwanted substancesPeroxisomes – membrane-walled sacs of oxidase enzymes

Enzymes neutralize free radicals and break down poisonsBreak down long chains of fatty acidsAre numerous in the liver and kidneys

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Cytoplasmic Organelles

Ribosomes – constructed of proteins and ribosomal RNA

Site of protein synthesisMay be free in cytoplasm, or associated with endoplasmic reticulum

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Cytoplasmic Organelles

Non-membrane bound organellesCytoskeleton – “cell skeleton” – an elaborate network of rods

Contains three types of rodsMicrotubules – cylindrical structures made of proteins, 25 nm in diameterMicrofilaments – filaments of contractile protein actin, 7 nm in diameterIntermediate filaments – protein fibers, 10 nm in diameter

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Cytoskeleton: Microtubule

Microtubules – found in cilia and flagella, centrosomes, spindle apparatus…

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Cytoskeleton: Microfilament

Figure 2.11b

Actin – forms the “thin filament” of muscle

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Cytoskeleton: Intermediate Filament

Figure 2.11c

Vimentin – common intracellular IF holding organelles in place

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Cytoplasmic Organelles

centrosomes and centrioles

Centrosome – a spherical structure in the cytoplasm

Composed of centrosome matrix (PCM – pericentriolar material) and centrioles

Centrioles – paired cylindrical bodies

Consists of 27 short microtubulesAct in forming microtubules for cilia, flagella, spindle apparatus

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Cytoplasmic Inclusions

Temporary structures – not present in all cell typesMay consist of pigments, crystals of protein, and food stores such as…

Lipid droplets – found in liver cell and fat cellsGlycosomes – store sugar in the form of glycogen

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The Nucleus

The nucleus –“central core” or “kernel” – control center of cell

DNA directs the cell’s activities

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Nuclear envelope – two parallel membranes separated by fluid-filled space

Pores allow for transfer of material3,000 – 4,000 pores in typical mammalian nucleus

~20,000 ribosomes/min can pass throughUp to 30,000 histones/min can pass through

Outer membrane continuous with the RER membraneChromatin – composed of DNA and histone proteins

Condensed chromatin – contains tightly coiled strands of DNAExtended chromatin – contains uncoiled strands of DNA

DNA's genetic code is copied onto mRNA (transcription)

The Nucleus

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The Nucleus

Chromosomes – highest level of organization of chromatin

Contains a long molecule of DNA

Nucleolus – “little nucleus” – in the center of the nucleus

Contains parts of several chromosomesSite of ribosome subunit manufacture

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Cellular Diversity

Specialized functions of cells relates to:Shape of cellArrangement, type and quantity of organelles(i.e. a muscle cell should have more mitochondria

than a simple skin epithelial cell)

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Cellular DiversityCells that connect body parts or cover organs

Fibroblast – makes and secretesprotein component of fibersErythrocyte – concave shape provides surface area for uptake of the respiratory gasesEpithelial cell – hexagonal shape allows maximum number of epithelial cells to pack together

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Cellular Diversity

Cells that move organs and body partsSkeletal, cardiac and smooth muscle cells

Elongated and filled with actin and myosinContract forcefully

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Cellular Diversity

Cells that store nutrientsFat cell (adipocyte) – shape is produced by large fat droplet in its cytoplasm

Cells that fight diseaseMacrophage – moves through tissue to reach infection sites

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Cellular Diversity

Cells that gather informationNeuron – has long processes for receiving and transmitting messages

Figure 2.18 (5)

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Cellular DiversityCells of reproduction

Oocyte (female) – largest cell in the body

Contains many copies of organelles for distribution to daughter cells

Sperm (male) – possesses long tail for swimming to the egg for fertilization

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Developmental Aspects of Cells

begin as a fertilized egg = ZygoteCells in embryo

Exposed to chemical signals Chemicals channel cells into specific pathways of development

Cell specialization leads to structural variation of cell types

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Developmental Aspects of Cells

Aging – a complex process caused by a variety of factors

Free radical theoryDamage from byproducts of cellular metabolismRadicals build up and damage essential molecules of cells

Mitochondrial theory – a decrease in production of energy by mitochondria weakens and ages our cellsNeurodegenerative disorders have been linked (recently) to nuclear pore aging. http://www.sciencenews.org/view/generic/id/40136/title/As_cells_age,_the_nuc

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Developmental Aspects of Cells

Genetic theory – proposes that aging is programmed by genes

Telomeres – “end caps” on chromosomesTelomerase – prevents telomeres from degrading